This invention relates generally to radio frequency circuits and more particularly to radio frequency switching circuits.
As is known in the art, radio frequency switches have many applications in radio frequency systems. One type of switching circuit well known in the art uses PIN diodes as passive switching elements. An example of such a switch, using PIN diodes, is described in a paper entitled "Microwave Switch and Attenuator Modules" by Reid, Microwave Journal, July 1973, pp. 145-148. PIN diode switches offer the advantages of moderate switching speeds, (i.e. of the order of tens of nanoseconds) and relatively good isolation generally exceeding 35 db over a relatively large frequency band. Nevertheless, several drawbacks exist with PIN diode switches. In particular, one drawback is that the PIN diode is not readily integrated with monolithic microwave integrated circuits. Many future radio frequency system requirements will specify monolithic microwave integrated circuits to reduce system size and cost while increasing system performance levels and reliability. Moreover, many system applications require faster switching speeds, generally less than 5 nanoseconds.
One solution to the problems of miniaturization and faster switching speeds is to provide passive switches based upon metal semiconductor field effect transistors (MESFETs). Such switches use field effect transistors, as passive elements, thus operating without output or drain circuit bias. These transistors are coupled in asymmetric series and shunt combinations to provide a particular switching circuit. One problem with this approach, however, is that heretofore relatively poor isolation has been provided by such switches. In general, isolation levels have been limited to less than about 35 dB over a frequency range of 2-18 GHZ. While such isolation may be tolerable for certain applications, in other applications, such as in electronic countermeasures less than 35 db isolation is inadequate. Often isolation levels exceeding 35 db are required. Thus, in those applications requiring a high degree of isolation, the PIN diode approach mentioned above, has been used. It would be desirable, nevertheless, to provide a FET based switch, which has a relatively high isolation and thus which can be integrated in a monolithic microwave integrated circuit.